Hydrophone cable

ABSTRACT

A hydrophone cable having a series of hydrophones (7) at spaced intervals along the cable and having strength members (1) and transmission line conductors (3) extending along the cable in which a braided construction (2, 3, 4 and 5) is used which can be loosened to insert the hydrophones (7) into cable, voids in the cable being filled with a buoyant gel and the assembly covered by an outer plastic sheet (6).

This invention relates to a hydrophone cable and in particular itrelates to certain improvements to cables of this type which are towedin the ocean and which are arranged as acoustic arrays comprising aseries of hydrophones spaced at required intervals and carried in abuoyancy device to allow towing from a vessel at a required depth belowthe surface of the ocean.

Such devices are used for seismic exploration and for ship and submarinedetection and similar purposes and operate on the basis that thehydrophones are coupled either singly or in groups to instrumentation onboard the vessel to act as sonar detectors.

The arrangement of these devices as known at present is such thatneutral buoyancy is achieved and according to forms of the device atpresent in use the hydrophones are spaced along a towing cable withwhich are also associated the transmission lines which transmit thesignals from the hydrophone to the vessel. The transmission lines (fordata and power), hydrophones and other components (preamplifiers, othersensors, spacers etc.) are contained in tubular pliable devices (casingsor sleeves) which are filled with a buoyant fluid such as kerosene sothat neutral buoyancy is achieved of the complete assembly. Theconfiguration is such that the required sensitivity of the hydrophonesis not lost because of their enclosure in the casing, the buoyant fluidfill having the required acoustic properties to transmit the soundpressure waves from the casing wall to the hydrophone elements.

Typical Prior Art Specifications are; U.S. Pat. No. 3,480,907 J. D. Kingwhich shows a hydrophone cable having hydrophones along its length atspaced intervals and including a jacket with tensile members along theinside of the outer sheath and teaching generally the art of neutralbuoyancy, also U.S. Pat. No. 3,696,329 G. D. Hazelhurst, U.S. Pat. No.3,518,677 E. F. Florian, U.S. Pat. No. 3,434,104 F. E. Stapleton et al,U.S. Pat. No. 3,319,734 G. M. Pavey, JR., U.S. Pat. No. 3,371,311 J.Colet et al, U.S. Pat. No. 3,531,760 W. A. Whitfill, JR., U.S. Pat. No.3,744,016 B. W. Davis, U.S. Pat. No. 3,978,446 G. Kirby Miller, andAustralian Pat. No. 457,052 Whitehall Electronics Corporation.

In the above hydrophone cables are shown having generally thecharacteristic of streaming cables which are generally of neutralbuoyancy, but thecost of such devices is relatively high and intricateand handling is a problem because of the long lengths usually involvedand the bulk of the units coupled with the need to be able to store thedevice on a vessel when not in use.

It is the object of this invention therefore to provide an improved formof acoustic array which will not have the intricacy of the previouslyused devices but which will be effective in picking up sounds and willbe as free as possible of noise normally caused through turbulence ofthe water around the array during towing.

The device according to this invention consists of a cable-likestructure constructed of polypropylene and/or similar materials whichtogether have the required buoyancy and tensile strength, and with whichare incorporated the spaced hydrophones and the leads for transmittingthe signal from the hydrophones along the array to the vessel, the cableof this invention being of braided construction such as by using a coreof polypropylene or of the material known under the trade name of"KEVLAR" and to wind around this in helical form the transmission cableswhich can be wound in a matrix of the buoyant material, a sheath beingextruded or otherwise placed over the whole assembly including thehydrophones which hydrophones can conveniently be of elongated form tofit into the cable, but the construction can be varied to have the hightensile members outwards of a core.

Because the structure can be built up of fibres forming cords wound intoa particular pattern it will be realised that for instance a series ofhydrophones can be spaced at required distances apart by untwisting thepolypropylene core at the area where they are to be inserted so thatuniform strength still exists over the complete length of the cable,additional winding then being applied in an appropriate manner whichincorporates the electrical leads from the hydrophones to the vessel.Either single leads for each of the hydrophones or arrays of hydrophonescan be included to give collective signals but, whatever thearrangement, it is possible to build up a cable which incorporates thehydrophones closely held in the buoyant material which, as said, beingin the nature of cords which are interwound, allows a requiredconfiguration to be attained, including the transmission lines and,because the cable is built up in layers which can be successivelyapplied around a basic core, the total cable can have a relativelyuniform diameter over its length.

Such a cable thus comprises a hydrophone cable having a series ofhydrophones at spaced intervals along the cable and having an outersheath and within it strength members and transmission line conductorsextending along the cable characterised by a braided construction suchthat the braided members can be loosened to insert the hydrophoneassemblies, into the cable. The transmission line conductors and othermembers are generally braided with fibre strands so that when somestrands are out at a hydrophone locality, at least the transmission lineconductors can be twisted to open to allow insertion of the hydrophoneassemblies.

In order however that the invention will be more fully understood,embodiments thereof will now be described with reference to theaccompanying drawings in which:

FIG. 1 is a perspective view of one form of cable, showing the cableprogressively stripped to show the braided construction,

FIG. 2 is a similar view of a modified form of cable,

FIG. 3 is a longitudinal somewhat schematic section of a cable at thehydrophone locality showing how a spacer surrounding the hydrophoneassembly allows the banding to extend around the hydrophone assemblyarea,

FIG. 4 is an exploded view of the hydrophone assembly components, and

FIGS. 5 and 6 are sectional views of the cable to a larger scale, FIG. 5being taken on line 5--5 of FIG. 3 and FIG. 6 being taken on line 6--6of FIG. 3.

In the form shown in FIG. 1 a series of "KEVLAR" strength members 1 havewound over them a braided polypropylene fill 2, over which in turn arewound layers of twisted-pair conductors 3 braided with polypropylenestrands 4 to form a cellular layer, over which is then formed an openpolypropylene braid 5.

The cable so formed is impregnated with a low specific gravity gel andpaper wrapped (not shown) before an outer sheath 6 is extruded over it.

The hydrophones 7 and preamplifiers 8 are inserted into the braidedstructure by opening up the braid as shown particularly in FIG. 4 and ifthe hydrophone assemblies are inserted into the cable after the braidingof the cable is completed, a boot 9 is placed over the area to replacethe sheath 6 where cut to give access to the braiding.

The reason why the braided structure can be opened up is that if thepolypropylene strands are cut at the area of the insertion, the layersof twisted pair conductors can be twisted to unwind the conductors intoa loose form where the hydrophone assemblies are to be positioned, afterwhich the conductors can be held outside of the hydrophone assemblies asillustrated herein.

It will be noted that in this form the "KEVLAR" strength members 1 formthe centre of the cable structure but are displaced outwardly around thehydrophone assemblies, the surrounding braided polypropylene coveringbeing also capable of being spread by cutting the braid in one directionand twisting the uncut members to insert the hydrophone assemblies.

The hydrophone assemblies can take the form shown in FIGS. 3 and 4 fromwhich it will be seen that the hydrophone 7 and preamplifiers 8 areinserted into an open-cell foam support 10 which in turn is insertedinto a rolled sheet-metal separator 11 and the two ends of this areclosed by a pair of plastic spreaders 12 and 13.

As seen in FIG. 3 the "KEVLAR" strength members and the braided strandsare placed around the outside of the separator as seen particularly inFIG. 6 and held by retaining bands 14 and 15 at the two ends of theassembly. The separator is as shown in FIG. 6, provided with grooves 16to locate the strength members 1 and with wider grooves 17 to house andlocate the conductors 3 which elsewhere are plaited with thepolypropylene fibres.

The outer sheath 6 is preferably formed of a tough plastic such aspolyethylene.

The filler can be layers of polypropylene braid to build up volume andhence buoyancy.

If required, an intermediate plastic sheath 18 can be extruded over theconductors directly as shown in FIGS. 2, 3 and 5 to provide additionalprotection against abrasion and water ingress, the interior then beinggel-filled prior to the extrusion of the said intermediate sheath.

In the form shown in FIG. 2 a braided open weave polypropylene core 20have sections of foam isolation 21 inserted at appropriate areas of thecore 20 in which the hydrophones 19 and preamplifiers (not shown) areinserted, and over this are positioned braided "KEVLAR" cord strengthmembers 22 held in place by an overlay comprising an open weave braidedstocking 23.

The layers of twisted pair conductors 24, braided with polypropylenestrands 25, are placed over the stocking 23 and over this is agel-filled open polypropylene braid 26. The cable is again gel-filledand wrapped before extrusion over it of the outer sheath 27.

In this form therefore, the "KEVLAR" strength members 22 are positionedaround a core 20 instead of forming a core as in the form described withreference to FIG. 1, but again it is possible to open up the cable atthe hydrophone localities by suitably cutting or distorting braidmembers running in one direction to allow the other members to betwisted to open a space sufficiently wide to allow insertion of thehydrophone assemblies. The core 20 will, of course, require to havesections cut out of it to allow insertion of the foam isolation members21 which will be coextensive with the core and held in place by the"KEVLAR" strength members 22 and the stocking 23 as well as by thebraided conductors 24.

From the foregoing it will be seen that a typical construction is to usea buoyant fibre cable which has a braided construction capable of beingloosened to insert the hydrophones and possibly a preamplifier for eachhydrophone, or a preamplifier for batches of hydrophones, and possiblyother sensors and electric/or electro-optic units for encoding, and toinclude over this area where the fibre cable has been disturbed cords of"KEVLAR" strength members and supporting construction members.

While the strength members are preferably formed of the material knownunder the trade name of "KEVLAR" other materials could be used. Anyvoids in the cable are filled with a buoyant gel by any known methodthat insures a void-free fill.

It will be realised from the foregoing that the construction accordingto this invention is highly advantageous with regard to strength andstructure generally in that the completed buoyancy cable can be ofsubstantially uniform diameter throughout its length, free fromobstructions and configuration which could cause a high noise level whenthe device is towed through water, the cable being readily coilable forstorage purpose on a vessel or the like.

It will be realised of course that while some forms of construction havebeen outlined above it will be possible to construct the acoustic arrayin many ways, the basis of the invention being the use of lightweightsynthetic materials having a strength such that they can replace thenormal steel cables or similar tension devices which because of theirweight and characteristic then require to have associated with themsubstantial buoyancy chambers to reach neutral buoyancy for the array,the preferred materials for this invention being, as said, materialssuch as polypropylene and that known under the trade name "KEVLAR" whichcan be stranded successfully and produce the necessary high tensilestrength, and the use of a buoyant gel in place of the normally usedbuoyant liquid such as kerosene, the gel, by its thixotropic property,improving the cable robustness by minimising both loss of buoyant filland water ingress should the cable become ruptured in use, and thecomponent conductors and synthetic fibres being configured in such a wayas to allow the insertion of packages containing sensors, preamplifiersand the like prior to impregnation with the buoyant gel and finally theextrusion of an outer plastic sheath.

I claim:
 1. A hydrophone cable having a series of hydrophones at spacedintervals along the cable and having an outer sheath and within itstrength members and transmission line conductors extending along thecable, the improvement comprising said cable having a braidedconstruction arranged so that the braided members can be loosened toinsert the hydrophone assemblies into said cable, said transmission lineconductors being helically wound in one direction and being braided withfiber strands helically wound in the opposite direction, said fiberstrands at spaced locations along said cable being cut whereby saidtransmission line conductors can be twisted open at said spacedlocations to receive a hydrophone assembly, a hydrophone assemblyinserted within said transmission line conductors at said spacedlocations, said outer sheath being extruded and filled with a buouantgel to give buoyance to the cable.
 2. A hydrophone cable according toclaim 1 wherein the said strength members form a core of the said cableheld in place by a braided overlay but extend outwardly to lie overhousings containing the said hydrophone assemblies.
 3. A hydrophonecable according to claim 1 wherein the said strength members extendalong the said cable outside of a central core and are held on to thesaid core by a braided overlay, and wherein the said core hascoextensive foam isolation members inserted in it to house the saidhydrophone assemblies.
 4. A hydrophone cable according to claim 1wherein the said hydrophone assemblies are held in an open-cell foamsupported in a tubular separator and having end spreaders, and the saidcable has a core of strength members carried out and over the saidtubular separator but held between the said separators by an overlay ofbraided members including twisted pair conductors braided with fibrestrands, said conductors being also carried out and over the saidtubular metal separators.
 5. A hydrophone cable according to claim 4wherein the said separator has longitudinal grooves to accommodate thesaid strength members and the said conductors.
 6. A hydrophone cableaccording to claim 1 or 4 wherein the said strength members are formedof a material known under the trade mark "KEVLAR" and wherein the saidbraided members excepting the said conductors are formed ofpolypropylene.
 7. A hydrophone cable according to claim 1 characterisedin that the cable is constructed in a continuous length without modularinterconnection.
 8. A method of forming a hydrophone cable having aseries of hydrophones at spaced intervals along the cable and having anouter sheath and within it strength members and transmission lineconductors extending along the cable, in which the cable is formed bythe steps of braiding, inserting at least sensors by cutting some of themembers of the braid other than the transmission line conductors,twisting the cable at the cut area to open the structure to allowinsertion of sensors connected to the transmission lines, closing thecable over the sensors, impregnating with a gel to give buoyancy to thecable, and extruding a sheath over the cable, whereby the sensors arepositioned at or near the core of the cable.